The present invention relates to a spring clutch comprising a clutch spring in the form of a coil spring mounted between an input member and an output member for transmitting the rotation of the input member in one direction to the output member and cutting off the transmission of rotation if the rotating speed of the output member exceeds the rotating speed of the input member.
Generally, with an engine accessory driving apparatus for transmitting the rotation of the crankshaft of an engine to rotary shafts of engine accessories through a belt transmission, if the engine is decelerated quickly, a pulley mounted on the rotary shaft of each engine accessory also tends to decelerate quickly. Especially in the case of an alternator, because the rotary shaft of the alternator has a large inertia, it cannot decelerate quickly in response to quick deceleration of the engine, so that the pulley mounted on the rotary shaft tends to continue to rotate at a constant speed.
In this state, there occurs a big difference between the speed of the pulley on the crankshaft and that of the pulley on the rotary shaft of the alternator, so that the belt tension increases and the belt becomes liable to break.
Also, the angular speed of the crankshaft changes in one turn. Such a change in angular speed causes slip between the belt and the pulleys, thus causing fatigue of the belt and impairing its durability.
Japanese patent publication 2003-322174A proposes a clutch pulley device which is free of this problem.
In this clutch pulley device, a cylindrical clutch spring made of a wire having a square section is mounted between a pulley and a pulley hub mounted in the pulley. The clutch spring has its outer periphery in elastic contact with a cylindrical clutch surface formed on the inner periphery of the pulley and has one end thereof in engagement with the pulley hub. When the pulley rotates in one direction, the clutch spring expands and increases the engaging force to the clutch surface, so that the rotation of the pulley is transmitted to the pulley hub through the clutch spring.
When the speed of the pulley hub exceeds the speed of the pulley, the clutch spring is radially compressed, causing slip between the clutch surface of the pulley and the clutch spring, so that the pulley can rotate freely.
With the clutch pulley device of Patent publication 2003-322174, since one end of the clutch spring is inserted and engaged in a spiral groove formed in one end of the pulley hub, a gap is formed between the outer periphery of the portion of the clutch spring and the clutch surface formed on the inner periphery of the pulley outside of the inlet of the spiral groove.
Therefore, in the deformation mode of the clutch spring in torque transmission, a bend is formed at a portion of the clutch spring corresponding to the inlet of the spiral groove and stress concentrates at the bend during torque input.
As a result, the clutch spring can break at its bend due to fatigue after repetition of loading of torque. This was a problem to be solved to improve durability.
An object of the present invention is to provide a spring clutch which has its durability improved by preventing the clutch spring from breaking due to fatigue.